1. Field of the Invention
This invention relates to radar and more particularly to phased array radar.
2. Description of the Prior Art
An antenna in one type of a phased array radar is comprised of 4480 radiating horns. Each of the horns provides a three thousand watt pulse of radiation that comprises a component of a radar beam. In the horn, the radiation is in a TE.sub.10 mode. Outside of the horn, the radiation is in a TEM mode.
The beam has a direction that is usually expressed as a pointing angle subtended from a central axis of the antenna. The pointing angle is determined by the respective phase angles of excitation of the horns. The excitation with desired phase angles is provided to the horns via phase shifters. Since the antenna is comprised of 4480 horns the radar includes 4480 phase shifters.
An undesired coupling of signals from the phase shifters is obviated by enclosing each of the phase shifters in a metal enclosure. Accordingly, the phase shifters and horns comprise 4480 subassemblies where each of the enclosures has a proximal surface that abuts the closed end of the horn.
Electromagnetic energy propogates in a subassembly along a path from a phase shifter to a horn through an interface therebetween. The interface is a discontinuity in the path. Because of the discontinuity, the horn may be excited in modes of orders that are higher than the TE.sub.10 mode. The excitation in the higher order modes affects the radar beam in an unpredictable manner.
The horn is on the order of twenty centimeters from its closed end to its open end. The twenty centimeter length is needed to provide a region in the horn where there is a decay of the energy in the higher order modes. Moreover, the twenty centimeter length provides for space near the open end of the horn where a pair of capacitive coupling irises and an inductive coupling iris are disposed to provide an impedance match between the horn and free space.
The subassembly typically has a length of thirty two centimeters from the distal non-abutting surface of the enclosure of the phase shifter to the open end of the horn. Since there are 4480 phase shifters and 4480 horns, the phased array radar is bulky and heavy.
The discontinuity is eliminated from the subassembly when the horn and the phase shifter are of an integrated construction. An integrated subassembly has not been constructed because of a belief that an undesired coupling between the horn and the phase shifter is inherent in the integrated subassembly.